Some projectiles are not being slowed down by cobweb

Some projectiles are not being slowed down by cobweb.

This bug was fixed in snapshot 15w49a, but as a result it was possible to use witches and snow golems to get unlimited items(MC-93685) and caused another bug(MC-93636) that removed the feature, which should only slow down projectiles instead of turning them into items, like bubble columns(MC-125757).

Affected projectiles:

1. arrow

2. spectral_arrow

3. trident

4. egg

5. snowball

6. ender_pearl

7. potion

8. fireball

9. small_fireball

10. dragon_fireball

11. shulker_bullet

12. experience_bottle

13. llama_spit

14. wither_skull

15. firework_rocket

16. fishing_bobber

17. wind_charge

18. breeze_wind_charge